Noise masking installation



Jan. 17, 1967 M.J. KODARAS ETAL 3,298,455

NOISE MASKING INSTALLATI ON Filed March 11, 1964 2 Sheets-Sheet 1 FIG Jz;

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'W/ n LV77777l H// i United States Patent 3,298,455 Patented Jan. 17,1567 ice - 3,298,455 NOISE MASKING INSTALLATION Michael J. Kodaras,Scarsdale, N.Y., and John F. Duda, Dumont, N.J., assignors to ElofHansson, Inc, New York, N.Y., a corporation of New York Filed Mar. 11,1964, Ser. No. 351,128 3 Claims. (Cl. 181-.5)

This invention relates to the art of sound masking and more particularlyto an installation for masking undesirable sounds of noises in a givenarea.

As conducive to an understanding of the invention, it is noted that inmany environments such as in private offices, hospital rooms, homes,theatres and the like, the ambient noise level, i.e., the noise levelcreated by many indeterminate sounds, is too low to preventobjectionable or undesired sounds from being heard. More particularlyfor purposes of illustration, in the situation where a waiting room isadjacent a private office, and the door between the two rooms is open,if the ambient noise level in the waiting room is too low, conversationsheld in a normal tone in such private office can readily be heard in thewaiting room.

To mask the undesired sounds in a given area, air conditioning orventilating devices present in the area may have their orifices oroutlets designed to provide noise, or noise generating devices may beinstalled in the ducts leading to such outlets, Where there are no airconditioning or ventilating devices, sound generating devices havingloud speakers associated therewith may be installed in the walls orceiling of such area to project sound directly into the'area.

All of these devices are inadequate for their intended purpose, fortheir sound is directional and can readily be located by the occupantsof the area in which the masking sound is generated so that it isdistracting to such occupants; the sounds generated frequently haveobjectionable frequency spectrum characteristics; or the devicesrequiring air flow suffer from lack of control to provide the propersound level required to mask the undesired noise or sound.

It is accordingly among the objects of the invention to provide arelatively low cost installation which utilizes substantiallyconventional components and which for a given area will provide a soundthat is non-directional in character; that is non-irritating to theoccupants of the area and which will mask undesired sounds.

More particularly, it is an object of the invention to provide aninstallation of the above type for use in areas such as waiting rooms orthe like in which it is desired to mask or render unintelligibleconversations being held in adjacent rooms without need for closing ofcommunicating doors between such rooms or of soundproofing the room inwhich the conversation is being held.

According to the invention, the area in which the sound is to be maskedto provided with a hollow wall or plenum chamber defined between one ofthe walls of the area, such as the ceiling of the building and anacoustical ceiling suspended therefrom, the latter being of partiallysound transparent material.

Positioned in the plenum chamber is a sound projecting device orientedto project the sound emitted therefrom away from the acoustical wall ofthe plenum chamber so that it will strike the building ceiling anddiffuse through the entire plenum chamber, thereby affordingsubstantially uniform distribution of the sound against the innersurface of said acoustical ceiling which will permit partialtransmission therethrough of such sound.

More particularly, the frequencies of the sound emitted by the soundprojecting device and the volume of such sound may be set depending uponthe characteristics of the acoustical ceiling, the acousticalcharacteristics of the room and the nature of the sound to be masked.

In the accompanying drawings in which is shown one of various possibleembodiments of the several features of the invention,

FIG. 1 is a top plan view of a typical installation according to theinvention,

FIG. 2 is a sectional view showing the building ceiling with theacoustical ceiling suspended therefrom and the sound projecting device,

FIG. 3 is a circuit diagram of a typical electrical sound generatingdevice, and

FiG. 4 is a chart illustrating frequency range and intensity level ofvarious sounds.

Referring now to the drawings, as shown in FIGS. 1 and 2, the soundmasking installation illustratively is incorporated in the waiting room11 of an office 12, for example, which have an intercommunicating door13 which is normally to be left open.

Suspended from the building ceiling 14 of the waiting room, as shown inFIG. 2, are a plurality of panels 15 of acoustical material such as thinplywood, wood fibre, gypsum board, plaster board, erf-orated metalacoustical ceilings or the like which panels 15 are also partially soundtransparent. As the acoustical ceiling may be suspended from thebuilding ceiling in any conventional manner,

such as for example, as shown in Patent No. 3,103,868,

the method of suspension will not be described as it is well known inthe art and per se forms no part of the invention.

As is clearly shown in FIG. 2, the space between the building ceiling 14and the acoustical ceiling 15 defines a plenum chamber 16'. In theillustrative embodiment shown, sound projecting devices such as one ormore loud s eakers 16 are positioned in the plenum chamber adjacent theacoustical ceiling 15. The mouth 17 of each speaker is directed upwardlyas shown, so that the sound emitted therefrom will strike the buildingceiling 14 and then be diffused through the entire plenum chamber 16 andagainst the entire inner surface of the acoustical ceiling.

Even though the masking sounds from the speakers 16 are initiallydirectional, due to the diffusion of the sound through the plenumchamber, the sound will pass uniformly through the partially soundtransparent acoustical ceiling and as the latter covers substantiallythe entire area of the room, the net effect will be non-directional tothe occupants of such room.

Although the sound emitted by the speakers 16 may be generated by anysuitable equipment, a typical device is shown in FIG. 3.

Thus, a transistor 21 which has an inherent noise level which isamplified, acts as the sound generating element and the noise voltage isdeveloped across the load resistor 22 of said transistor 21. Resistor 23and capacitor 24 act as a filter circuit for the supply voltage totransistor 21.

Transistor 25 which is a conventional amplifier stage, is coupled totransistor 21 through capacitor 26; the base resistor 27 providingstability, the resistor 28 being the collector load resistor andresistor 29 providing bias to the base of transistor 25.

The transistor 31 forms part of an emitter follower and is directlycoupled to transistor 25, the resistor 32 serving as the load resistorfor such emitter follower. The output of the emitter follower is coupledthrough capacitor 33 to transistor 34 of another amplifier stage inwhich resistor 35 is the emitter resistor and diode 36 and resistor 37form the collector load.

Transistors 39, 41 form a push-pull output stage in which directcoupling to transistor 41 and between diode 36 and resistor 37 totransistor 39 provides bias stability and signal coupling.

The resistor 42 serves to bias transistor 34 which in turn biasestransistors 39, 41 and provides degenerative feedback for stability.

Capacitors 33, 45 control the frequency spectrum by limiting the lowfrequency response through capacitor 33 and by passing the highfrequencies through capacitor 45, thereby providing proper frequencyspectrum characteristics.

The values of capacitors may be selected based on the frequency responseto suit the particular system.

Capacitor 46 feeds the output signal to the loud speakers 16 throughvariable resistor 47 which may be set to adjust volume.

In the operation of the system to mask out the sound of normalconversation in an ofiice adjacent a waiting room, referring to FIG. 4,the typical speech signal may have a curve shown at A, i.e., at lowerfrequencies its sound pressure level in db is low, then such pressurelevel rises to a maximum at say 300 cycles per second and then fallswith increase in frequency.

The ambient noise in the waiting room may have the curve shown at B inFIG. 4, i.e., at low frequencies, the sound pressure level is high andwould mask low frequency speech signals. However, at the major portionof the conversation range, i.e., from approximately 220 c.p.s. to 3,800c.p.s., the sound pressure level of such ambient noises would beinsuflicient to mask the speech signal.

Accordingly, the sound emitted by the speaker is adjusted in frequencyand volume so that when heard in the waiting room it would follow thecurve shown at C in FIG. 4, which curve is a composite of the ambientnoises and the sound emitted through the acoustical ceiling.

Thus, the sound represented by curve C has a frequency range and soundpressure level such that it will override or mask the speech signalpassing into the waiting room thereby rendering such speech signalunintelligible to the occupants of the waiting room.

With the installation above described, occupants of a private oflice maytalk freely without likelihood of their conversations being overheard byoccupants of an adjoining waiting room even when direct and unimpededcommunication exists therebetween.

As many changes could be made in the above installation, and manyapparently widely different embodiments of this invention could be madewithout departing from the scope of the claims, it is intended that allmatter contained in the above description or shown in the accompanyingdrawings shall be interpreted as illustrative and not in a limitingsense.

Having thus described our invention what we claim as new and desire tosecure by Letters Patent of the United States is:

1. An installation for masking undesired sounds in a chamber having twoadjacent areas defined by an intervening wall having an opening, each ofsaid areas having a ceiling, said installation comprising a plenumchamber extending over one of said ceilings, said one ceiling definingthe lower wall of said plenum chamber, said plenum chamber having anupper wall spaced from the lower wall, said lower wall having its outersurface exposed to the interior of the area associated with the plenumchamber and its inner surface exposed in said plenum chamber, said lowerwall being of partially sound transparent material, a sound generatorpositioned in said plenum chamber and mechanically isolated fromsaidlower wall, said sound generator comprising a sound projectingdevice and means to create a sound to be emitted by said soundgenerating device, said sound having a frequency spectrum that isbetween predetermined ranges which is substantially equal to thefrequency range of the sounds to be masked, said sound projecting devicebeing oriented to project the sound generated toward the upper wall ofsaid plenum chamber, whereby the sound projected by the sound projectingdevice will diffuse through the entire plenum chamber affordingsubstantially equal distribution of such sound against the inner surfaceof the lower wall of the plenum chamber, said lower wall permittingpartial transmission therethrough of such sound, thereby masking theobjectionable sounds.

2. The combination set forth in claim 1 in which means are provided tosuspend said lower wall from said upper wall, to define said plenumchamber.

3. The combination set forth in claim 2 in which said sound projectingdevice comprises a loud speaker positioned in said plenum chamber withthe mouth of said loud speaker directed upwardly toward said upper wall.

References Cited by the Examiner UNITED STATES PATENTS 811,140 1/1906Garney 18133.03 2,001,812 5/1935 Biasi 1810.5 X 3,104,543 9/1963Kaminski 181-05 X 3,229,429 1/1966 Conard 181-05 X BENJAMIN A. BORCHELT,Primary Examiner. M, F, HUBLER, Assistant Examiner.

1. AN INSTALLATION FOR MASKING UNDESIRED SOUNDS IN A CHAMBER HAVING TWOADJACENT AREAS DEFINED BY AN INTERVENING WALL HAVING AN OPENING, EACH OFSAID AREAS HAVING A CEILING, SAID INSTALLATION COMPRISING A PLENUMCHAMBER EXTENDING OVER ONE OF SAID CEILINGS, SAID ONE CEILING DEFININGTHE LOWER WALL OF SAID PLENUM CHAMBER, SAID PLENUM CHAMBER HAVING ANUPPER WALL SPACED FROM THE LOWER WALL, SAID LOWER WALL HAVING ITS OUTERSURFACE EXPOSED TO THE INTERIOR OF THE AREA ASSOCIATED WITH THE PLENUMCHAMBER AND ITS INNER SURFACE EXPOSED IN SAID PLENUM CHAMBER, SAID LOWERWALL BEING OF PARTIALLY SOUND TRANSPARENT MATERIAL, A SOUND GENERATORPOSITIONED IN SAID PLENUM CHAMBER AND MECHANICALLY ISOLATED FROM SAIDLOWER WALL, SAID SOUND GENERATOR COMPRISING A SOUND PROJECTING DEVICEAND MEANS TO CREATE A SOUND TO BE EMITTED BY SAID SOUND GENERATINGDEVICE, SAID SOUND HAVING A FREQUENCY SPECTRUM THAT IS BETWEENPREDETERMINED RANGED WHICH IS SUBSTANTIALLY EQUAL TO THE FREQUENCY RANGEOF THE SOUNDS TO BE MASKED, SAID SOUND PROJECTING DEVICE BEING ORIENTEDTO PROJECT THE SOUND GENERATED TOWARD THE UPPER WALL OF SAID PLENUMCHAMBER, WHEREBY THE SOUND PROJECTED BY THE SOUND PROJECTING DEVICE WILLDIFFUSE THROUGH THE ENTIRE PLENUM CHAMBER AFFORDING SUBSTANTIALLY EQUALDISTRIBUTION OF SUCH SOUND AGAINST THE INNER SURFACE OF THE LOWER WALLOF THE PLENUM CHAMBER, SAID LOWER WALL PERMITTING PARTIAL TRANSMISSIONTHERETHROUGH OF SUCH SOUND, THEREBY MASKING THE OBJECTIONABLE SOUNDS.